1. Field of the Invention
The present invention relates generally to the field of biomass gasifiers, and more particularly to biomass gasifiers producing combustible gases with low tar contents as needed for as needed for running internal combustion engines.
2. Description of Related Art
A variety of systems are known for combusting biomass materials to generate heat and power. Three classes of gasifiers are common in the prior art: updraft gasifiers, downdraft gasifiers with nozzles for air injection, and downdraft gasifiers without air injection nozzles.
In updraft or countercurrent systems, combustion air enters below the char bed, and fuel enters at the top. The combustion air first encounters ash and glowing char. Oxygen within the combustion air is consumed. The char bed temperature reaches its peak at the point downstream where the oxygen is generally completely consumed. Hot gaseous combustion products, primarily CO2 and H2O, react endothermically at temperatures between about 90 degrees C. and about 1200 degrees C. with the hot char according to the reactions
C+CO2xe2x86x922CO 
C+H2Oxe2x86x92CO+H2. 
thus producing hot fuel gas and converting energy contained in the temperature of the hot gas and char materials into usable fuel gases, including H2, CO, and CH4, at temperatures greater than about 700 degrees C. Below 700 degrees C., the reaction rates are much slower, and the hot gas primarily preheats incoming fuel. As wood and other biomass fuels are heated, they are first dried and then decomposed by pyrolysis into three classes of components: condensibles, char, and gas. Because no oxygen is available at the point where tar is produced, virtually all tars released during pyrolysis pass out of the updraft gasifier with the gas. Typical tar contents in updraft are about 30 percent (300,000 ppm). Such gas is burned directly and is not suitable for engine operation.
While up to a million vehicles were operated on gas fuel that was produced from wood during WW II, these gasifier engine systems provided inadequate tar cleanup. Tar accumulation within the vehicle engines and tar disposal remained a significant obstacle to commercialization. Existing updraft gasifiers that are fueled by dry wood or other dry, unpyrolyzed biomass fuels produce output gas containing typically 30% (300,000 ppm) tars. Downdraft gasifiers with nozzle air introduction, such as those of the Imbert or Hesselman type, and downdraft gasifiers utilizing stratified bed flaming pyrolysis, without nozzles, produce output gas having relatively lower but still substantial tar contents, generally in the range of 1000 ppm to 3000 ppm. Further, the use of a plurality of nozzles, orifices, and/or tuyeres, such as are found in an Imbert gasifier, undesirably causes a plurality of alternating and localized hot spots and cool spots to be formed respectively at the locations of the nozzles and at locations between the nozzles. These hot spots operate to destroy tars, but they undesirably also worsen the tendency of ash slag and clinkers to form. Undesirably, tars pass through the cool spots, and no destruction of hot tar destruction occurs in these cool spots. As a result, the tar levels in these prior gasifiers are typically between 1000 ppm and 3000 ppm.